This protocol contains three components, each assigned a separate Duke IRB number. The first component utilizes 131Iodine-labeled 81C6 monoclonal antibody in the treatment of patients with neoplasms metastatic to the leptomeninges. The purpose of this study is to determine the safety and maximum tolerated dose of intrathecal 131I-labeled 81C6 monoclonal antibody in patients with neoplasms metastatic to the leptomeninges. The second component utilizes 131I-labeled 81C6 monoclonal antibody in the treatment of patients with primary or metastatic malignant brain tumors with surgically created cystic resection cavities. The purpose of this study is to determine the safety and maximum tolerated dose of 131I-labeled 81C6 monoclonal antibody administered into surgically created cystic tumor resection cavities in patients with primary or metastatic malignant brain tumors. The third component utilizes 131I-labeled 81C6 monoclonal antibody in the treatment of patients with recurrent cystic gliomas. The purpose of this study is to determine the safety and maximum tolerated dose of 131I-labeled 81C6 monoclonal antibody administered into naturally occurring tumor cyst cavities in patients with recurrent cystic gliomas. These disease entities are all devastating, incurable neurologic complications of cancer where present treatments are inadequate. The development of monoclonal antibodies has provided the potential for more specific therapy of tumors which are reactive with the monoclonal antibody or antibody fragment. Antibodies that are specific to the tumor cells and that do not react with normal brain or spinal cord can be conjugated with therapeutic radioisotopes, such as 131I. This conjugate can then be delivered intrathecally for leptomentingeal neoplasms, into a naturally-occurring tumor cyst cavity for cystic brain tumors, or into a surgically-created resection cavity for solid brain tumors to deliver a therapeutic dose of radiation with relative specificity for the tumor cells. Radiolabeled monoclonal antibodies may be a significant new therapeutic modality for these disease entities. These are the first studies in the United States to test such compartmental therapy of leptomeningeal neoplasms and brain tumor resection cavities with radiolabeled monoclonal antibodies.